23

Glacial Lakes in the Himalayas:
A Review on Formation and Expansion Processes
Akiko SAKAI
Graduate School of Environmental Studies, Nagoya University
F3-1(200), Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan*e-mail: shakai@nagoya-u.jp

Abstract
In the recent several decades, glacial lakes have been expanding at the terminus of debris-covered
glaciers associated with glacier shrinkage in the Himalayas. Accordingly, glacial lake outburst floods(GLOFs) have become serious natural disasters in the Himalayas. Bathymetrical surveys have been carriedout at several glacial lakes, and the relationship between the lake area and water volume has been illustrated.
At the initial stage, several small lakes are distributed near the glacier terminus, and then they coalesce and become one large lake, and finally, the large lake expands rapidly through calving of the glacial front. Surface
inclination, surface velocity and surface lowering of glaciers are significant indicators of glacial lake expansion. Future prediction of topographical changes in debris-covered glaciers is important in forecasting theformation and expansion of glacial lakes.
Key words: ablation processes, calving, debris-covered glacier, glacial lake area, water volume

1. Introduction
There are many debris-covered glaciers in places suchas the Himalayas, Andes, Italian Alps and Southern Alpsin New Zealand. These debris-covered glaciers contains
many glacial lakes in ablation areas (e.g., Lliboutry et al.,
1977; Vallon, 1989; Kirkbride, 1993; Clague & Evans,
2000; Diolaiuti et al., 2006; Gardelle et al., 2011).
In the Himalayas, large valley glaciers several
kilometers long occupy about 80% of the glacialized
areas (Moribayashi & Higuchi, 1977) and have mostly
debris-covered ablation areas. The fronts of debriscovered glaciers advanced down to approximately
4,500 m a.s.l. during the Little Ice Age. Some glaciallakes have started to expand at the terminus ofdebris-covered glaciers in the 1950s and 1960s (Yamada,
1998; Komori, 2008). Glacial lakes are surrounded anddammed by terminal- and lateral-moraines which formed
during the Little Ice Age, and others are dammed bystagnant ice masses at the glacial front. The danger
glacial lakes pose depends largely on the relative elevation from the base of the surrounding moraines to the lake
surfaces (Benn et al., 2001; Sakai et al., 2007; Fujita
et al., 2008).
Moraine dams generally fail on account of overtopping surges and incision by outlet streams. This overtopping is caused by large waves generated by huge masses
falling into the lake, such as snow or glacier avalanches;or by moraine failures induced by earthquakes; or by
Global Environmental Research
16/2012: 23-30
printed in Japanadvancing, sliding and calving (ice wastage caused byshedding of large ice blocks from a glacier’s edge into thebody of water) of the mother glacier (Ives, 1986;
Vuichard & Zimmermann, 1987; Yamada, 1998; Iwata
et al., 2002). Melting of ice cores in moraines and piping
are other possible failure mechanisms (Clague & Evans,
2000). Thus, outburst floods are caused by breaking of
the moraine damming glacial lakes due to the above
reasons, and these events are called glacial lake outburst
floods (GLOFs). At the beginning of the last century
the number of GLOFs increased in the Himalayas(Richardson & Reynolds, 2000).

2. Measurements of Glacial Lakes
2.1 Bathymetric measurement of glacial lakes
The first series of the depth distribution measurements in the Himalayas was carried out at Abmachimai
Co, Qangzonk Co, and Paqu Co in the Tibetan Plateau by
LIGG / WECS / NEA (1988). In the Nepal Himalayas,
Imja Glacial Lake was measured in 1992 by Yamada andSharma (1993), and in 2002 by Sakai et al. (2003). The
Tsho Rolpa, Lower Barun and Thulagi lakes have been
also surveyed (Yamada, 1998). Recently, the Tsho Rolpa,
Thulagi and Imja lakes were surveyed again in 2009 by
ICIMOD (2011). Raphstren Tsho and Lugge Tsho in the
Bhutan Himalayas were measured by the Geological
Survey of India (1995) and Yamada et al. (2004),
respectively. The location, area, water volume and maxi©2012 AIRIES24

Akiko SAKAI

Table 1 List of locations, areas, volumes and maximum depths of glacial lakes, for which bathymetrical surveys have been carried out.
Volume ×106
2

Longitude Area (km )(m3)

Maximum
depth (m)Lake name

Surveyed Date

RegionLatitudeType

Reference

Abmachimai Co

Apr, 1987

Tibet

28°05'29"N

87°38'17"E

0.56

19.00

72 moraine-dammed glacial lake

LIGG/WECS/NEA (1988)

Qangzonk Co

Apr, 1987

Tibet

27°55'36"N

87°46'15"E

0.76

21.40

68 moraine-dammed glacial lake

LIGG/WECS/NEA (1988)

Paqu Co

Apr, 1987

Tibet

28°18'09"N

86°09'34"E

0.31

6.00

36 moraine-dammed glacial lake

LIGG/WECS/NEA (1988)

Lower Barun

Mar, 1993

Nepal

27°47'49"N

87°05'52"E

0.60

28.00

109 moraine-dammed glacial lake

Yamada (1998)

Imja

Apr, 1992

Nepal

27°53'58"N

86°55'18"E

0.60

28.00

99 moraine-dammed glacial lake

Yamada (1998)

Imja

Apr, 2002

Nepal

27°53'58"N

86°55'18"E

0.90

36.00

91 moraine-dammed glacial lake

Sakai et al. (2003)

Imja

May, 2009

Nepal

27°53'58"N

86°55'18"E

1.01

35.50

97 moraine-dammed glacial lake

ICIMOD (2011)

Feb, 1993 & Feb, 1994 Nepal

27°51'57"N

86°28'24"E

1.39

76.60

131 moraine-dammed glacial lake

Yamada (1998)

Tsho Rolpa
Tsho Rolpa

Aug-Sep, 2009

Nepal

27°51'57"N

86°28'24"E

1.54

85.94

134 moraine-dammed glacial lake

ICIMOD (2011)

Thulagi

Mar. 1995

Nepal

28°29'45"N

84°28'47"E

0.76

31.80

81 moraine-dammed glacial lake

Yamada (1998)

Thulagi

Jul. 2009

Nepal

28°29'45"N

84°28'47"E

0.94

35.37

80 moraine-dammed glacial lake

ICIMOD (2011)

0.50

10.00

- moraine-dammed glacial lake

Mool et al. (2001)

Dig Tsho-Nepal

27°52'25"N

86°35'06"E

Tam Pokhari(Sabai Tsho)

-

Nepal

27°44'28"N

86°50'39"E

0.47

21.25

- moraine-dammed glacial lake

Ngozumpa

Oct. 1999

Nepal

27°57'13"N

86°42'01"E

0.05

0.24

12

thermokarst lake

Benn et al. (2000)

RipimoNov. 1993

Nepal

27°53'24"N

86°28'15"E

0.02

0.18

25

thermokarst lake

personal communicationfrom Dr. T. Kadota

Raphsthren

1984, 1986

Bhutan28°06'14"N

90°14'45"E

1.38

66.83

88 moraine-dammed glacial lake

Geological survey of India(1995)Lugge

Sep-Oct. 2002

Bhutan28°05'39"N

90°17'44"E

1.17

58.30

126 moraine-dammed glacial lake

Yamada et al. (2004)

Mool et al. (2001)

mum depth of the glacial lakes contacting glacier ice aresummarised in Table 1. The relationships between areasand volumes of glacial lakes are shown in Fig. 1. The
lake volume (V: ×106 m3) and lake area (A: km2) havefollowing relationship:
V = 43.24×A 1.5307
Huggel et al.(2002) has also represented similar relationships from glacial lakes located in the Swiss Alps,
including ice-dammed lakes.
The relationship between the area and maximum
depth of glacial lakes is shown in Fig . 2. The relationshipbetween the maximum depth of lakes (Dmax: m) and lakeareas can then be calculated as follows,

Fig. 1 Relationship between the areas and water volumes ofglacial lakes in the Himalayas listed in Table 1.

Dmax = 95.665×A0.489
The observed glacial lakes shown in Table 1 are incontact with glacier ice at the higher ends of the lakes,
and the deepest points are located at upstream positionsof almost all glacial lakes. Bathymetric re-measurements
have been carried out only at Imja glacial lake by Sakai
et al. (2003) and Fujita et al. (2009). The results showthat as the glacial lake expands due to the glacier front
retreating, the lake bottom near the terminal moraine
undergoes little change. Takenaka et al. (2010) carried out
electrical resistivity tomography near the terminal moraineof the Imja Glacial Lake. They concluded that dead ice
remained under a thick debris layer, and only gradual deadice melt occurred due to insulation by the thick debris.

Fig. 2 Relationship between the areas and maximum depthsof glacial lakes listed in Table 1.

A Review on Expansion Processes of Glacial Lakes
Figure 3 shows changes in Tsho Rolpa Glacial Lake
between 1994 and 2007. The pictures were taken from themountain slope downstream. Some small islets near theterminal moraine in 1994 have disappeared or shrunk in
2007, although the water level of the lake was lowered by
2-3 m in 1999 by mitigative construction works (Reynolds,
1998). The reason of the islets disappearance would be theslow melting of dead ice under the thick debris layer. For
this reason, it is necessary to assess the effect of dead ice
melting in these glacial lakes, which would decrease thestrength of the terminal moraine damming the lake water.

2.2 Records of glacial lake areal expansion
Expansion of glacial lakes has been recorded bytopographic map and satellite images. Mool (1995),
Yamada (1998), Ageta et al. (2000), Komori et al. (2004)
and Komori (2008) have collected available maps orsatellite images showing areal expansion of each glacial

25

lake at different dates and summarized surface area
expansion of glacial lakes in the Nepal and Bhutan
Himalayas. At the initial stage, several small lakes were
distributed in the ablation areas near the glacier terminus.
Then those lakes coalesced and formed large lakes.
Shore-lines at the upstream sides of such lakes tend to bestraight lines, indicating expansion of the glacial lakes bycalving of retreating glacier fronts. On the other hand, thechange in shore-lines on the downstream side is relatively smaller than upstream (Watanabe et al. 2009).
Komori (2008) reported that the expansion rate of a
glacial lake on the southern side of a debris-covered
glacier in the Bhutan Himalayas was higher than that on
the northern side, and the expansion on the northern sidestarted earlier than that on the southern side. Gardelle
et al. (2011) selected seven study sites along theeast-west wide-spread mountain range from the Bhutan

Fig. 3 Tsho Rolpa Glacial Lake, taken in June 1994 by Dr. N. Takeuchi (Chiba University) (upper)
and on Nov. 4, 2007 by Mr. Osafumi Sato (The Asahi Shimbun) (lower).

26

Akiko SAKAI

Himalayas to the Hindu Kush and mapped glacial lakes
with LANDSAT satellite imagery acquired in 1990, 2000
and 2009. They reported that in the east part (India, Nepal
and Bhutan), glacial lakes were bigger and more
numerous than in the west, and have continuously grown,
while glacial lakes have shrunk during that period in the
Hindu Kush and in the Karakoram. This areal differenceof glacial lake change corresponds to less glacier ice loss
in the western Himalayas and mountains further west.

3. Elaborate Ablation Processes ofDebris-covered Glaciers
Mattson et al. (1993) summarized the ablation rate ofice under the debris layer with various thicknesses, andreported that glacier ice covered with thick debris (morethan 0.5 m) does not melt because of insulation by the
thick debris. Generally speaking, retreats and advances ofdebris-free glaciers can be used as indicators of glacier
response to climate change. While, in the case of debriscovered glaciers, frontal changes do not represent glacier
response to climate changes since change in theirterminus areas is controlled by the thick debris covers(Scherler et al., 2011).
At the Khumbu Glacier, the thickness of the debris
layer increased gradually from just beneath the icefall of
the glacier to the terminus, where the thickness of debris
up to 5 km from the glacier terminus reached more than
one meter (Nakawo et al., 1986). It can, therefore, be
estimated that the Khumbu Glacier would not undergo
melting or downwasting of the lower part in terms of theexperimental relation shown by Mattson et al. (1993)
because the thickness of the debris covering the lower
part was larger than 0.5 m. However, actually, the glaciersurface has lowered 5-18 m from 1978 to 1995 according
to Kadota et al. (2000) and the estimated annual ablation
rate using thermal resistance from satellite data (Nakawo
et al., 1993) is more than one meter (at most 3 m)
(Nakawo et al., 1999). Inoue and Yoshida (1980), Sakai
et al. (1998, 2002) and Röhl (2008) reported that the melt
rate of ice cliffs surrounding supraglacial lakes was muchhigher than that of areas of ice covered with thick debris.
Thus the relatively high melt rate at the lower part of the
Khumbu Glacier, covered with a debris-layer of morethan one meter in thickness, can be explain by rapid
ablation at the ice cliff surface and the presence ofsupraglacial lakes.
As for supraglacial lakes themselves, Kirkbride(1993) inferred that supraglacial lakes aligned along an
englacial conduit may be created by the collapse of the
conduit roof. Actually, Gulley and Benn, (2007), Gulley
et al. (2009), Benn et al. (2009) have directly observed
entrances or insides of conduits connected with englacialconduits at the Khumbu Glacier. Furthermore, Sakai et al.
(2000) suggested that the roof of the conduit could collapse, leading to the formation of ice cliffs and ponds,
which would accelerate the ablation of the debriscovered glacier. Iwata et al. (2000) has supported theabove presumption, indicating that large relief areas,

occupied by a lot of ice cliffs and supraglacial ponds,
expanded both upward and downward from the middle
parts of the Khumbu Glacier ablation area from 1978 to1995.

4. Supraglacial Lake Expansion
Kirkbride (1993) has summarized glacial lake expansion processes in terms of topographical views based on
his observation at the Tasman Glacial Lake in New
Zealand. He concluded that 1) debris-covered ice melts
slowly under the debris mantle, 2) collapse of conduit
roofs perforates the supraglacial mantle and allows more
rapid melting of bare-ice walls, and 3) lakes coalesce andincrease in depth by bottom calving with a potentiallyunstable ice floor. Röhl (2008) also indicated that subaqueous calving occurred in supraglacial lakes of the
Tasman Glacier, and 4) disintegration of the ice floor
increases water depth and initiates a rapid calving retreat.
In the case of the Himalayas, terminal moraines are
ice-cored, and glacier ice near the terminus may be filledwith meltwater, since ice-cored moraines have lesspermeability, as seen in 1) in Fig. 4 (Yamada et al., 2002).
Figure 4 shows a schematic diagram of glacial lake
expansion processes, taking into account the dead ice inthe terminal moraine, as modified by Kirkbride (1993).
Kirkbride (1993) and Röhl (2008) focused on thetransition from melting to calving glacier termini, sinceFig. 4 Schematic diagram of the glacial lake expansion
process on debris-covered glaciers, indicating thelongitudinal cross section of the glacier terminus. (1)Slow melting under thick debris layer. Glacier surface
is still high. (2) Growth of lakes by calving from thelake bottom. (3) Initial rapid lake expansion by calvingof glacier front during retreat.

A Review on Expansion Processes of Glacial Lakes

the onset of calving at glacier termini represents thecommencement of rapid expansion of glacial lakes. Most
calving research has focused on tidewater glaciers (Vander Veen, 1996, 2002; Vieli et al., 2001), but lake-calving
has received much less attention (Benn et al., 2007).
Glacial lakes expand rapidly due to glacier ice calving,
therefore, calving processes have to be studied to establish a glacial lake expansion model.
It was found that glacier ice calving has been causedby flotation of glacier ice at several tidewater and lacustrine glaciers, such as the Mendenhall Glacier (Motyka
et al., 2002; Boyce et al., 2007) and Bering Glacier(Lingle et al., 1993) in Alaska, Rhonegletscher in
Switzerland (Tsutaki et al., 2011), Upsala Glacier(Naruse & Skvarca, 2000; Skvarca et al., 2003) andGlacier Nef (Warren et al., 2001) in Patagonia. Meanwhile, recently several studies have found that thermoerosional notch growing rates at the waterline areimportant determinants of the rate-controlling mechanism during sub-aerial calving (Kirkbride & Warren,
1997; Benn et al., 2001; Warren & Kirkbride, 2003;
Diolaiuti et al., 2005; Röhl, 2006, 2008; Benn et al.,2007). Röhl (2006), who has directly measured thermal
undercutting at the Tasman Glacial Lake, concluded thatthe calving rate was directly controlled by the rate ofthermal undercutting, and implied that the processes ofthermal undercutting play a central role in the transition
from mere melting to calving termini. Warren andKirkbride (1998), Benn et al. (2001) and Haresign and
Warren (2005) have also observed thermal undercutting
rates. The above results by several researchers suggestthat thermal-undercutting of subaqueous glacier ice acts
as a trigger for lacustrine calving.
In the high Himalayan mountains, valley winds arestrong during the daytime (Inoue, 1976; Chikita et al.,1997, 1999). The average daily maximum wind velocity
can reach more than 10 m s-1 during the melt season (Juneto September) (Yamada, 1998). To examine the contribution of water currents driven by valley winds in thevariable topography of pro-glacial lakes, to the onset ofcalving due to thermal undercutting, Sakai et al. (2009)
calculated wind velocity distributions according to varying fetches, as well as wind-driven lake surface currents.
The results suggest that the onset of calving due to thermal undercutting is controlled by water currents driven
by winds at the surface of the lake, which develop in
proportion to expanding water surface. Chikita (2007)
calculated the wind velocity distribution over the lake
surface at Tsho Rolpa and Imja Glacial Lake. He concluded that the wind speed over the lake surface at the
Imja Glacial Lake was less than that at Tsho Rolpa due tothe difference in end moraine heights, which act as an
obstacle in the path of valley winds. Actually the rate of
expansion of the lake at Imja (40 m yr-1) was less thanthat at Tsho Rolpa (70 m yr-1) from the 1960s to the 1990s(Yamada, 1998). The difference in rates of expansion
between the Imja and Tsho Rolpa lakes is due to thedifference in wind speeds over the lakes.

27

5. Formation Conditions of Glacial Lakes
Reynolds (2000) reported that in the Bhutan
Himalayas, glacial lakes form in those parts of the glacierwhere the inclination of the glacier surface is less than 2°.
Quincey et al. (2007) reported that glacial lakes develop
upon debris-covered glaciers with low slopes and a surface speed less than 10 m a–1. Suzuki et al. (2007) calculated the thermal resistance of debris-covered glaciersand found that those with relatively thin debris layers
tend to develop glacial lakes at their terminus. This finding suggests that glaciers with high rates of ablation tend
to develop glacial lakes. In summary, the available evidence indicates that glaciers that record a relatively large
lowering of their surface are likely to develop glaciallakes at their terminus. Indeed, Kirkbride (1993) reported
that lowering of the glacier surface resulted in theformation of supraglacial ponds and the commencementof calving at the glacial lake. Lamsal et al. (2011) havereported that the glacier surface declined about 100 m
during the Imja Glacial Lake expansion from several
small lakes in 1964 to a large lake in 2006.
Usually, the ablation rate reached a maximum at theglacier terminus in the case of debris-free glaciers. In
general, though, large glaciers are covered with debris at
the lower part, and the thickness of the debris increases
from equilibrium line altitude, reaching more than one
meter at the terminus of the glacier. Thus, ablation rates
at the terminus are negligibly small and increase up to
several kilometers from the terminus. After reachingablation maximum at the middle part of the ablation area,
ablation rates decrease with altitude due to decreasing air
temperature. Therefore, small surface lowering at theglacier terminus and large surface lowering at the middle
part of the ablation area reduce inclinations at the lower
part of debris covered glaciers.
Naito et al. (2000) calculated the longitudinal glaciersurface change of the Khumbu Glacier by establishing a
new model coupling the mass balance of debris-covered
glaciers and glacier flow models. The model predicted
the formation of depressions in the lower ablation area,leading to glacial lake formation in the depressions.
Bolch et al. (2008) confirmed intensive surface loweringby comparing digital elevation models produced from
Corona with observations from ASTER (Advanced
Spaceborne Thermal Emission and Reflection radiometer) at the Khumbu Glacier. Nuimura et al. (2011)
analysed glacier surface elevation changes using digital
elevation models in 1978, 1995 and 2004, which weretaken by field observations, and they detected a remarkable acceleration of surface lowering in the middle part of
the ablation area at the Khumbu Glacier.
Sakai and Fujita (2010) identified the formation
conditions of glacial lakes by analysing two easily
measurable topographic parameters: inclination of theglacier surface and the difference in relative height
between the glacier surface and lateral moraine ridges(herein, this difference is referred to as DGM, which is anindicator of glacier surface lowering on debris-covered

28

Akiko SAKAI

glaciers). They concluded that for all glaciers with large
supraglacial lakes, values of DGM exceed 60 m and theaverage inclination of the glacier surface is less than 2.0°.
Low inclination indicates relatively small ice flux to thelower part of the glacier, which induces glacier shrinkage,
and a large DGM represents the proximity of the glaciersurface to the water level in glacier ice, which yields
stable supraglacial lakes as shown in Fig. 4.
Benn et al. (2001) believed that the Khumbu andNgozumpa glaciers were close to the threshold formoraine-dammed glacial lake formation. Those two
glaciers definitely have low inclinations but there havebeen no appropriate indicators for the possibility offormation of glacial lakes. Consequently, glaciers withlakes and without lakes can be clearly distinguished bytaking into account not only inclination but also DGM in
the Nepal and Bhutan Himalayas as stated by Sakai and
Fujita (2010).

6. Concluding Remarks
Debris-covered glaciers both with lakes and withoutlakes are found under identical meteorological conditions,for example, at the Khumbu Himal glacier, and the
Lunana glacier in the Bhutan Himalayas (Fig. 5). What isthe difference between these glaciers with lakes and without lakes? Twenty years before, we did not have ananswer to this question, but now we can give some explanations. In the past two decades, formation and expansion processes have been elucidated.
Glacial lakes expand on a debris-covered glacier,which has low inclination (little ice flux from upstream)
and large surface lowering. Both formation conditions ofglacial lakes are caused by one phenomenon, glaciershrinkage. Glacier shrinkage is caused by less precipitation or large ablation. Ablation processes of debriscovered glaciers are more complicated than those ofdebris-free glaciers. Not only there is a debris mantle ofvarious thicknesses and sizes, but also unstable supraglacial lakes, which constitute heat absorbing spots ondebris-covered glaciers, preventing us from establishinga model of ablation processes of debris-covered glaciersat present. In the next twenty years of research on
Himalayan debris-covered glaciers, however, we should
concentrate on the study of ablation processes of debris-covered glaciers taking into account that of rapidchanging supraglacial lakes in order to predict fluctuations of debris-covered glaciers and formation of glaciallakes.

Acknowledgements
I would like to express thanks to Mr. Osafumi Sato
(The Asahi Shimbun), Dr. Nozomu Takeuchi (Chiba
University) and Dr. Koji Fujita (Nagoya University) for
offering photographs. I would also like to also express
thanks for considerate reviews and comments from Prof.
Shuji Iwata and the editor, Prof. Teiji Watanabe.
This study was supported by SATREPS (Science andTechnology Research Partnership for Sustainable
Development), which is a joint program between JST(Japan Science and Technology) and JICA (Japan
International Cooperation Agency), and NEXT program(Funding Program for Next Generation World-Leading
Reserchers).
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Akiko SAKAI
Akiko SAKAI is an Assistant Professor at theGraduate School of Environmental Studies,
Nagoya University. She has been studying glaciallakes in the Himalayas since 1993, and has also
carried out observations of the ablation process ofdebris covered glaciers since 1995. She has
carried out field observations at Tsho Rolpa and Imja Glacial Lakes and at
Lirung and Khumbu Glaciers in the Nepalese Himalayas, and hydrologicalobservations at Qilian Shan in China, and is studying the discharges fromglaciers in arid regions. At present, she is working on developing a GlacierInventory of Asian Mountains.

(Received 25 October 2011, Accepted 13 February 2012)